Adequate control over the concentration of radioactive particulate material in breathing air is essential in the responsible and acceptable operation of nuclear fuel facilities. Radioactive contamination of the air is sometimes caused by aerosols containing actinide bearing particles, typically a combination of one or more of the .alpha. emitting isotopes: .sup.238 Pu, .sup.239 Pu, .sup.240 Pu, .sup.241 Am, .sup.242 Cm and .sup.244 Cm, together with the .beta. emitting actinide .sup.241 Pu.
The classical method of monitoring airborne radioactivity is to sample the air, filter out any particulate matter in the air, and then monitor the radioactive emissions from the filter. The trouble with monitoring actinide radioactivity in the air is that the permissable radioactivity levels are so low as to be masked by the natural background airborne .alpha. radioactivity, particularly from the decay products (daughters) of radon gas. The radon daughters which typically interfere with .alpha. emission detection of actinides are the following: .sup.218 Po (RaA), .sup.214 Po (RaC'), .sup.216 Po (ThA), .sup.212 Bi (ThC) and .sup.212 Po (ThC'). The maximum permissable actinide radioactivity concentration for air (MPC.sub.a) as laid down by International Commission on Radiological Protection (ICRP) is typically between 3 and 6 dpm m.sup.-3 (1.4 to 2.8.times.10.sup.-12 uCi cm.sup.-3), depending on the isotope mix. The safety rules of the Central Electricity Generating Board (CEGB) in England and Wales define a Contamination Zone Class CIII (into which persons entering must wear approved full face respiratory protection) as an area where, averaged over a period not exceeding 13 weeks, airborne radioactivity concentrations are expected to exceed one tenth of the ICRP recommended maximum (MPC.sub.a) for a forty week occupational exposure. Accordingly, in order to monitor a region to ensure it remains below the lower limit requiring respiratory protection, the monitoring apparatus must be capable of sensitivities to the actinides of typically between 0.3 and 0.6 dpm m.sup.-3. By comparison, under normal conditions the concentrations one metre above ground level of natural radioactivity resulting from .sup.222 Rn (radon) plus .sup.220 Rn (thoron) range from 400 to 4,000.times.10.sup.- 13 uCi cm.sup.-3. The above CEGB lower limit for which respiratory protection must be used is called the Derived Working Level (DWL).
Statistical uncertainties make it impossible to monitor the instantaneous radioactivity level and in practice average values over extended periods of time are determined. If the airborne actinide concentration is not to average above 1 DWL during a working shift to say 8 hours, this is equivalent to measuring that the actinide concentration does not exceed 8 DWL-hours by the end of an 8 hour shift.
The techniques for monitoring actinide concentration in the air which has been adopted previously in most cases is to sample the air and then wait 72 hours before monitoring the .alpha. emissions from the sample. The .alpha. emitting radon daughters from natural background radiation have relatively short half lives, typically a few minutes, compared to the half lives of the actinides (half a year to several thousand years). Accordingly the above technique permits the natural radon and thoron daughter products to decay to practical insignificance before monitoring the sample to detect the remaining .alpha. emissions which should be representative of actinides. This delay of 72 hours can, however, pose operational problems since additional precautions must be maintained in case there should be a relatively sudden increase in actinide concentration which would not be detected by previous methods for three days. Accordingly breathing protection must often be used even when, in reality, airborne radioactivity levels are in fact much lower than the lower limit for the zone requirements (e.g. CEGB Class CIII). Furthermore, existing techniques actually obtain an average of the contamination over an eight hour shift and provide the results of this monitoring only three days later. If an unexpectedly high count is in due course observed, it is often difficult to determine the particular part of the operation during the shift three days previously which might have been responsible.
There are various proposals in the prior art for actinide monitoring apparatus and methods which provide, at least at some extent, "on-line" monitoring. In "The ZPR-9 Airborne Plutonium Monitoring System", IEEE Transactions on Nuclear Science, Vol. NS-23, No. 1, February 1976, by Rusch, McDowell and Knapp, an on-line monitor is disclosed which samples the air but discriminates against solid particles which have less than a certain minimum aerodynamic size (1.5 microns). This separation is based on the observation that the radon and thoron daughter products are normally associated with dust particles which are generally substantially smaller than the actinide containing particles.
The apparatus described in this prior art article then provides some further rejection of remaining radon and thoron daughter activity in the collected sample by distinguishing between the characteristic energies of the resulting emissions. The described apparatus employs a silicon detector which provides an output pulse dependent on the energy of the detected .alpha. emission. In the described example the energy resolution of the detector is about 300 keV.
Separate scalers are arranged to count pulses from the detector in two different energy windows, one representing .alpha. energies between 5 and 5.4 MeV and the other representing energies between 5.5 and 7 MeV. In this way the count of pulses representing emissions having energies in the first window is a count primarily of .alpha. emissions from the decay of .sup.239 Pu, rejecting most of the .alpha. emissions from decay of the radon and thoron daughter products, which have energies in the range from 6 MeV up to nearly 9 MeV.
Because some .alpha. particles lose some of their energy before being detected by the silicon detector, a proportion of higher energy .alpha. particles are nevertheless detected in the low energy window and the described apparatus cancels these out by recording also the number of counts in the high energy window and subtracting a fixed proportion of these from the count in the lower energy window. Because the low energy window of the described apparatus extends only between 5 and 5.4 MeV, the apparatus measures primarily only the radioactivity resulting from .sup.239 Pu and .sup.240 Pu, which have characteristic energies in this range. The apparatus is not responsive to certain further actinides which can be important constituents of airborne radioactivity particularly resulting from the fuel cycle of thermal nuclear power stations.
Furthermore, the apparatus described in this prior art article has a sensitivity sufficient to enable it to be set to alarm if airborne plutonium concentrations exceed about 2.times.10.sup.-11 .mu.Ci-hr cm.sup.-3 (10 RCG-hr using the notation of the publication). This corresponds to approximately 100 DWL-hr (using the notation referred to previously), which implies that an airborne radioactivity concentration in excess of 1DWL could exist throughout a 48 hour working week without causing the monitor in the prior art document to alarm. Clearly the monitor described in this publication would be unsuitable to meet the safety rules set down by the Central Electricity Generating Board.
Still further, the described apparatus is arranged to collect particulate matter from the air directly on a greased surface of the .alpha. emission detector. As a result the apparatus cannot be left unattended for an extended period of time since the detector must be cleaned and re-greased each morning.
A further prior art publication of interest is "A Review of Measurement Techniques for Stack Monitoring of Long Lived .alpha. Emitters", IEEE Transactions On Nuclear Science, Vol. NS-26, 757 (1979). This publication reviews various techniques for stack monitoring of long lived .alpha. emitters and refers briefly to the above described ZPR-9 monitoring system. An alternative arrangement for discriminating against small particles in the sampled air is described, constituting a two stage virtual impactor. The virtual impactor system deposits the larger size particles on a filter paper immediately in front of the detector, instead of directly on the detector face as described previously.
This second article then goes on to describe an alternative monitoring system called "The Transuranic Aerosol Measurement System (TAMS)". This system does not perform any particle separation by size in the sampled air. All particles in the sampled air are deposited on a section of a strip of filter paper. The filter paper extends between feed and take-up spools and when a sufficient sample is collected on a particular section of the paper strip, the strip is wound on to bring the sample into a detection chamber. The detection chamber is then sealed and evacuated before .alpha. emissions from the sample are monitored and counted. The evacuation of the detection chamber substantially improves the spectral resolution of the different energies of .alpha. emissions from the sample. In this way the various .alpha. emitters in the sample can be better distinguished from one another by their different characteristic energies, and in particular .alpha. emissions from actinides can be more readily distinguished from .alpha. emissions from background radon and thoron daughter products. The articles also refers to the possibility of "decay scheme analysis" to eliminate residual, natural .alpha. background which might not otherwise be rejected by the energy discrimination. In particular the .alpha. emissions from .sup.218 Po might be difficult since their characteristic energy is about 5.99 MeV which is close to the characteristic energies of certain actinides. The decay scheme analysis mentioned in the article would be based on the difference in half lives between the long lived .alpha. emitters (the actinides) and the relatively short half life of .sup.218 Po.
In the event, the article states that sufficient resolution is obtained simply from energy resolution so that decay analysis was not required or performed. The sensitivity of the system described in this second article enables it to measure a concentration of 0.25 DWL from a 60 minute sample with a fractional standard deviation of 18%. This is very much more sensitive than the ZPR-9 apparatus described previously but there are certain difficulties in using the described system.
The need to evacuate the detection chamber adds substantial complication to the apparatus. The evacuation requirement is crucial to the high energy resolution performance of the described device. However there is a tendency for particles to be detached from the filter paper during evacuation and to contaminate the face of the detector. Additionally, it is of critical importance that the filter medium used in the described system is of extremely high quality so as to prevent any substantial number of collected particles from penetrating into the medium. A very expensive filter medium is required to maintain good resolution and this would indicate a cost of operation of around 1,000 per month.
The described system is also particularly adapted for monitoring .alpha. emitters in stack effluents, positioned downstream of the stack's own filters. These stack filters are designed to remove particulate material from the effluent down to very small sizes with great efficiency. As a result a very large proportion of the dust particles on which radon and thoron daughter products may have aggregated have already been removed by these filters before the sampling point. The system would not operate with as much success in a different environment with the normal concentration and size range of dust particles and hence the normal radon and thoron background.